Abstract
The primary aim of this study was to investigate the effects of seed inoculation with the biocontrol agent Pseudomonas fluorescens strain F113 (producing the antibiotic DAPG) and its modified derivative strain F113G22 (with DAPG production disrupted) on the uptake of nitrogen by pea and wheat plants. Uptake of N by the two plant species was investigated in soil microcosms amended with 15N-labelled fertilisers (urea or ammonium nitrate) or 15N-labelled plant residues. Uptake of fertiliser-N was unaffected by inoculation. However, uptake of N derived from organic residues was enhanced in pea inoculated with either strain. In contrast, uptake of N by wheat was reduced in the presence of either strain F113 or F113G22, suggesting that the effects of these microbial inocula on N-mineralisation in the rhizosphere were dependent on plant species. It was subsequently found that microfaunal populations, especially soil nematodes in the rhizosphere of inoculated pea were significantly larger than those associated with the rhizosphere of non-inoculated controls. In wheat, however, microfaunal populations in the rhizosphere of inoculated plants were lower than those associated with noninoculated controls. These trends were repeated using simple sand microcosms into which soil bacteria and the bacterial-feeding nematode Caenorhabditis elegans were introduced. This suggested that effects on N-mineralisation were mediated by changes in populations of microbial-feeding microfauna. As a possible explanation for the increased nematode populations in the rhizosphere of inoculated pea plants, the nematicidal effects of pea seed exudates on C. elegans were investigated in small-scale sand systems. It was found that exposure to non-inoculated pea seeds reduced the short-term survival of C. elegans as compared to unamended sand, and that survival was greater in the presence of pea seeds inoculated with either strain than non-inoculated seeds, suggesting that nematicidal compounds released by germinating pea seeds were utilised by the P.fluorescens strains. No such effects were observed for wheat.